Portable, low-power, label-free, real-time, compact, high-sensitivity, high-throughput, and cost-effective assay tools are highly attractive and can be widely used in environmental monitoring, homeland security, biomedicine, biochemistry and pharmacy. A label-free sensor is an assay tool that enables direct biochemical and chemical detection, and it is generally desirable due to its non-intrusive nature of detection different from the sensor with labelling with compounds (e.g. fluorescent, radioactive, and colorimetric).
Optical sensors [1] for biochemicals or chemicals are a kind of label-free sensors using light as the detection mechanism. They comprise optical transducers to convert the presence or the amount of chemical or biological agents into quantitatively measurable optical signals, such as phase, amplitude, and frequency. These optical sensors have many advantages, such as less complexity due to no labelling, in situ real-time monitoring, and high sensitivity.
There are several optical sensors for biochemical or chemical sensing with noticeable success, for example based on surface plasmon resonances [2], input/output grating couplers [3,4,5], evanescent wave devices [6], and guided mode resonance (GMR) [7]. Although optical sensors using these methods are very sensitive with discrete optical detection units, they are still bulky and expensive. Optical sensors based on surface plasmon resonances require both a discrete optical source and an optical detection unit. Grating-coupler-based optical sensors have inherent difficulties, due to their operation principle, to simultaneously integrate the optical source and the optical detector. Optical sensors based on evanescent waves, such as ring resonators, require expensive external tunable diode lasers. GMR optical sensors use a discrete detector to monitor the peak shift in the reflectivity spectrum.
Infinite-size high-contrast gratings (HCGs) always have a broadband high reflectivity, and can serve as mirrors. Finite-size HCGs demonstrate a Fano resonance with a very high Q value, and a kind of HCG-based biosensor was proposed [8]. This HCG-based biosensor, like above-mentioned optical sensors, is very bulky, expensive and hard to handle, because it requires a discrete optical detection unit to monitor the peak shift in the reflectivity spectrum.
TDLAS (Tunable Diode Laser Absorption Spectroscopy) requires expensive laser diodes of very special wavelengths and of very high cost. The required optics for higher sensitivity are very costly. TDLAS works well in the gas-phase but has severe problems with liquids.